When cells formed respectively by a plurality of base station apparatuses overlap with each other, interference between downlink signals that are transmitted by these cells sometimes becomes a problem. For example, in a mobile communication system in accordance with Long Term Evolution (LTE) defined by the 3rd Generation Partnership Project (3GPP), interference to a Physical Downlink Control Channel (PDCCH) sometimes becomes a problem. This is because a mobile station device demodulates a signal received from a base station apparatus based on control information contained in the PDCCH.
It is possible to reduce the interference by differentiating, for each of the plurality of base station apparatuses, a time period in which a downlink signal from the each base station apparatus arrives at a mobile station device. As one example of such a technique, there is an inter-cell interference control technique called enhanced inter-cell interference coordination (eICIC) introduced in Long Term Evolution (LTE).
In the eICIC, a macrocell schedules a subframe that does not contain data called an almost blank subframe (ABS). A picocell or a femtocell transmits a downlink signal in a time slot in which an ABS is scheduled.
As a related technique, there is a method of synchronizing base stations in a wireless communication system where adjacent base stations perform transmission at the same frequency. The step of synchronizing a first base station with a second base station is carried out through a mobile station. The synchronization includes a step of causing the mobile station to listen to synchronization signals from both the base stations and to measure a difference between results of receiving the synchronization signals from both the base stations, a step of causing the mobile station to synchronize with the first base station and to transmit data from the mobile station so that the data is received by the first base station with a given offset, and a step of causing the second base station to measure the offset of the data received from the mobile station (for example, refer to Japanese Laid-open Patent Publication No. 2000-315978).